1. Field of the Invention
This invention generally relates to the field of arc welding, and in particular to a system and method for aligning and guiding a welding head relative to a workpiece along an operator defined weld path, without the need for constant operator intervention.
The system and method of the invention replaces purely mechanical systems in which the weld path cannot easily be changed, as well as systems that permit the weld path to be changed by manual intervention during welding. By using the system and method of the invention, an operator may pre-set a guide path and vary the path as desired by selecting coordinates of discrete points along the path, the final path being provided by interpolation between adjacent points on the path.
Despite automating guidance of the weld head or torch during welding, the system and method of the invention does not require automatic sensing of the weld path through mechanical contour probing or the like, and therefore can easily be implemented by adding a simple remote control interface and steering motor or, in semi-automatic welding apparatus that already include a pendant, control station, or other remote control operator interface and a cross-seam drive motor, by modifying the control software. Because of its simplicity, the system and method of the invention can be implemented in a wide variety of arc welding systems, and can provide variation of the weld path along an arbitrary number of degrees of freedom.
2. Description of Related Art
It is well-known to remotely control welding operations by manually varying such parameters as arc length and cross-seam oscillation amplitude as the weld head is moved along a fixed path relative to the workpiece.
While the fixed path of conventional weld head guiding arrangements is in general adequate for simple orbital, circular, or linear weld paths, it is difficult to adjust the weld path during welding, requiring stoppage of the welding process and/or constant operator intervention to ensure that the weld head follows the desired path.
Adjustment of the weld path during welding is particularly difficult where the welding is carried out in harsh environments, such as in the presence of radioactivity or elevated heat, because of the difficulty in accessing the weld head during welding, and in situations where obstructions exist that inhibit or eliminate visual contact with the welding arc. Operation of a welding apparatus requires a considerable amount of skill due to the number of variables associated with the welding process, and adding weld head guidance to the tasks that the operator must perform only complicates the operator""s task, making it more difficult to achieve a quality weldment. As a result, provision of a weld head self-guiding system and method according to the principles of the present invention would assist in the ease of operation and production of quality weldments in nearly all circumstances.
Conventional welding apparatus types which would benefit from automatic weld head guidance and alignment are semi-automatic orbital weld head, circular seam welding, and plane welding apparatus. For example, semi-automatic orbital weld heads are conventionally arranged to follow a path defined by a guide track as they orbit around a pipe or tube, but it is difficult to align the guide track with the intended weld line, and if the weld line does not follow a near perfect plane, some sort of operator intervention is necessary. On the other hand, in automatic weld heads used for welding circles, such as seal welds on tank lids, a central pivot system is used which causes the weld head to inscribe a near perfect circle even though the component to be welded may be eccentric relative to the torch path, or noncircular. Similar problems of workpiece irregularity exist for flat guide tracks used in plane welding.
The need for accurate guiding of the weld head is particularly critical in the case of semi-automatic GMAW or GTAW welding, with or without oscillation of the weld head, where an accurate root pass is critical in order to ensure that the land faces are fully fused/consumed. When a bevel or groove is being filled, the torch must be positioned to ensure proper bead-placement and sidewall-fusion without excessive build-up on either side of the weld. In order to apply conventional track guiding in such a system, frequent manual verification and adjustment of the track position is required, which is both time-consuming and expensive, or a complex contour sensing system such as the one disclosed in
U.S. Pat. No. 4,935,597, in which contour probing in conjunction with a weld torch swivel mounting is required, greatly increasing the cost and complexity of the welding apparatus.
It is accordingly a first objective of the invention to provide a system and method for aligning and automatically guiding a weld head along an operator-defined path without the need for operator intervention during welding.
It is a second objective of the invention to provide an automatic weld head alignment and guidance system and method arranged to be easily adapted for use in existing arc welding apparatus, including semi-automatic orbital weld head apparatus, circular seam welders, and plane welders.
It is a third objective of the invention to provide a simple operator interface through which an automatic weld head guide path may be defined and varied, and an intuitive method of using the interface.
It is a fourth objective of the invention to provide an automatic weld head alignment and guidance system and method which may be implemented through an existing semi-automatic weld head pendant, control station, or remote control interface utilizing an existing steering motor, or which may alternatively be retrofitted onto a welding apparatus that lacks such a suitable pre-existing steering motor and remote interface.
These objectives are achieved, in accordance with the principles of a preferred embodiment of the invention, by providing a system and method in which the operator is able to preset a welding path by establishing coordinates along the weld line to maintain the torch on the center of the weld line, the system and method of the invention including an interpolating or curve-fitting function that establishes a smooth path between the coordinates established by the operator so as to follow the weld line without major steps or discontinuities.
In particular, the system of the invention includes a first weld head assembly drive arranged to move the weld head assembly in a travel direction along a fixed path, and a second steering drive, which may include one or more steering motors, for steering the weld head or a welding torch relative to the fixed path in order to guide the weld head or torch along a desired welding path relative to the fixed path in response to control signals from a controller that has been pre-programmed with the desired welding path. The weld head assembly drive may be a conventional drive motor arranged to move the weld head assembly along a track, or it may take the form of a robot arm arrangement, a pivot arm, or any other guidance mechanism for moving the weld assembly along the fixed path. The steering drive, on the other hand, may be a cross-seam drive motor such as the cross-seam drive motor conventionally used to cause cross-seam oscillation of the welding torch.
In accordance with the principles of the method of a preferred embodiment of the invention, programming of the desired weld path is carried out, by establishing a zero coordinate and moving the weld head along the first predetermined path while manually controlling the steering motor or motors to move the weld head to desired positions along the actual weld path, and recording the coordinates of the desired positions by storing the values of position encoders when the weld head is at the desired positions, the path being completed by interpolation between adjacent coordinates.
Interpolation of the path may be reduced in many cases, such as the case of an orbital pipe girth weld, to a simple slope calculation for a single axis offset function carried out by a single steering motor, such as the cross-seam drive motor conventionally provided in semi-automatic orbital welding apparatus. The slope curve can be calculated in terms of travel degrees of arc between points for a degree based weld head, or in terms of travel distance along a fixed path for a travel speed and time based weld head. The number of offset increments between preset points can then be calculated to an increment as small as the cross seam drive system is capable of, or set at a fixed number.
For the specific example of a semi-automatic welding apparatus having a travel motor and a cross-seam drive system, establishment of the zero coordinate may be carried out by having the operator initially position the weld head at a desired starting location by manually releasing a travel clutch and pushing/pulling the weld head to the desired travel location and re-engaging the same clutch, and/or by depression of the travel/steering jog buttons if available until the weld head is positioned to the desired starting location. The tungsten tip for GTAW or the filler wire for GMAW, GMAW-P, FCAW, or FCAW-P may be brought in close proximity to the workpiece and/or temporarily contacted with the workpiece to facilitate visual alignment. Alternatively, some sort of measuring device such as a camera including a lens indexed with cross-hairs or an alignment grid, could be used to further ensure accurate alignment of the weld head with the desired start location. Cameras would be especially useful for remote alignment, particularly in visually restricted or biologically hazardous areas, in which case the cameras could be mounted either on the weld held or locally.
When the weld head is located at the desired start location the operator depresses a button on the remote pendant or control station normally used during execution of a program while in either the test or the weld mode to control such functions as cross-seam oscillation amplitude, thereby zeroing encoder counters for each axis (steering and travel), thus establishing the fixed start location. The operator then uses the jog buttons to move the weld head to a next location and presses the button that was used for zeroing encoder counters to establish the next location as a coordinate by recording the encoder values for the next location, and continues to move the weld head to establish new coordinates until sufficient coordinates have been established to follow the desired path. Additional coordinates can be established, or coordinates can be deleted, after a path is tested in a similar manner to that described above.
When the coordinates of the path have been established, a start or resume sequence is initiated, and a program line is encountered that initiates travel, the system will interpolate steering between adjacent coordinates to provide a smooth route or course for the weld head to track. Steering is adjusted in fine steps in accord with the steering points continuously interpolated between any two adjacent coordinates. This steering process will occur as long as a path exists, allowing its use for multiple passes.
In an especially preferred embodiment of the invention, the system permits steering overrides to be generated from the remote pendant, the steering override causing the entire path to move (offset) in the direction of the override in predefined increments, with the revised path being stored immediately upon execution of an override and retained upon de-energizing the system.